Recovering From a Disaster

Disaster tolerance is the ability of a system to restore an application on
a secondary cluster when the primary cluster fails. Disaster tolerance is based on
data replication and failover. The Geographic Edition software enables disaster tolerance by redundantly
deploying the following:

Highly available clusters that are geographically separated

Data replication at either the host or the storage level

Backups and restoration and data vaulting

Data replication is the process of continuously copying data from the primary cluster
to the secondary cluster. Through data replication, the secondary cluster has a recent
copy of the data on the primary cluster. The secondary cluster can be
geographically separated from the primary cluster.

Failover is the automatic relocation of a resource group or device group from
a primary cluster to a secondary cluster. If the primary cluster fails, the
application and the data are immediately available on the secondary cluster.

The Geographic Edition software supports two types of migration of services: a switchover
and a takeover. A switchover is a planned migration of services from the
primary cluster to the secondary cluster. During a switchover, the primary cluster is
connected to the secondary cluster and coordinates the migration of services with the
secondary cluster. This coordination enables the data replication to complete and ensures that services
can be transferred from the primary cluster to the secondary cluster without loss
or corruption of data.

A takeover is an emergency migration of services from the primary cluster to
the secondary cluster. A system administrator can initiate a takeover to recover from
a disaster. Unlike a switchover, the primary cluster is not connected to the
secondary cluster during a takeover. Therefore, the primary cluster cannot coordinate with the
secondary cluster to migrate the services. Because of this lack of coordination, the risk
of data loss and data corruption in a takeover is higher than it
is with a switchover. The Geographic Edition software uses dedicated recovery procedures during
a takeover to minimize data loss and data corruption.